Background

On December 13, 2005, a local health department (LHD) notified the Indiana State Department of Health (ISDH) about several family members who had developed gastroenteritis, characterized primarily by diarrhea and vomiting, following a holiday buffet. A total of 93 individuals ate at the party. The meal was prepared by a local caterer.

Epidemiologic Investigation

The LHD and the ISDH initiated a cohort study to describe the outbreak and to determine whether the source may have been food related. The LHD obtained a menu of all food items served and a listing of party attendees. The ISDH developed a questionnaire which documented illness history, foods eaten, and other risk factors. LHD staff members conducted interviews and completed questionnaires for both ill and well individuals who attended the party, then forwarded completed forms to the ISDH field epidemiologist. A case was defined as any previously healthy person who ate at the party and became ill with diarrhea and/or vomiting on or after the date of the buffet.

Forty-one people were reported ill, with 39 out of the 93 attendees meeting the case definition. The attack rate was 42 percent. One individual was hospitalized due to severe gastroenteritis symptoms and released the next day. The cases ranged in age from 1 to 78 years, with the mean age being 27 years. The predominant symptoms included: diarrhea (84%), vomiting (76%), nausea (70%), body aches (54%), and chills (54%). The incubation period ranged from 6-72 hours, with the median being 24 hours (Figure 1). Fifty interviews were completed on those who were not ill.

Based on the analysis of these data, no food item was statistically associated with illness. Several ill attendees reported secondary cases within their households, which would support the hypothesis of a disease agent that could also be transmitted person to person as well as through food. During the interviews, it was discovered that two families in attendance had children who experienced diarrhea at the party. One family reported having similar symptoms a week prior to the party. Stool samples were collected from three cases and forwarded to the ISDH Laboratory (see Laboratory Results).

Environmental Investigation

A representative from the LHD visited the caterer and serving site to review food preparation practices. No critical violations were noted. Leftover food samples were collected and submitted to the ISDH Laboratory for analysis (see Laboratory Results). None of the caterers, cooks, or servers reported having any illnesses. The caterer also supplied food for other parties on the same evening with no illnesses being reported from the other functions. Two of the caterers submitted stool samples for analysis.

Laboratory Results

Enteric samples from three cases and two caterers were submitted to the ISDH Laboratory for analysis. All tested negative for Campylobacter, E. coli O157:H7, Salmonella, and Shigella. The three ill case samples tested positive for Norovirus by use of reverse transcription-polymerase chain reaction (RT-PCR) assays. The stool samples from the caterers were negative for Norovirus.

Bacteriological tests were conducted for coliforms, E. coli, and Salmonella. However, no foodborne pathogens were identified in the food samples submitted.

Conclusion

The causative agent of this outbreak was identified as Norovirus. Three cases were laboratory confirmed, and the illness was compatible among all cases. Norovirus (family Caliciviridae) comprises a group of viruses known to cause acute gastroenteritis in humans. Noroviruses are highly contagious, with an infective dose as low as 100 viral particles. Noroviruses are transmitted primarily through the fecal-oral route, either by direct person-to-person spread or through fecally contaminated food or water. Common symptoms include nausea, vomiting, and diarrhea, which resolve in about 24-48 hours. However, infected individuals can shed virus up to two weeks after symptoms cease.

These viruses are relatively stable in the environment and can survive freezing and heating to 140 degrees Fahrenheit. The main symptoms experienced were diarrhea and vomiting, with a mean incubation period of 24 hours and duration of symptoms lasting 24-72 hours. This is consistent with a viral pathogen such as Norovirus.

Foodborne viral outbreaks can occur when an infected individual with inadequately washed hands handles food that is served raw or ready to eat (e.g., salads, vegetable trays, cookies, etc). In addition, touching surfaces or sharing contaminated food utensils or drinks can lead to virus transmission. However, this outbreak was more likely caused by person-to-person transmission rather than foodborne transmission. The food handlers tested negative for Norovirus, but family members reported that sick children attended the party. In addition, one family reported that family members had been ill with similar symptoms during the week before the party. Therefore, it was likely that various attendees were infected during the party, and transmission was likely facilitated by close contact among party attendees. The food was served in a self-serve buffet, which would make it easier to share contaminated food items. Schools and nursing homes in the county were reporting similar cases of disease, an indication that gastroenteritis was circulating in the community prior to the event. Therefore, the individuals who became ill may have consumed food which became externally contaminated at the dinner buffet or from person-to-person transmission among other families attending the party.

In general, most viral outbreaks can be prevented by strictly adhering to the following food safety practices:

Thoroughly wash hands with soap and water before preparing food, after using the restroom, before eating, and after changing diapers or assisting someone who is ill. Since some people may continue to shed the virus for several weeks after symptoms resolve, strict hand washing is important in preventing the spread of the virus.

Wash all fruits and vegetables prior to serving, especially if served raw.

Stay home if ill with diarrhea and vomiting. Food handlers should be excluded from work until symptoms cease.

Sick children and ill infants in diapers should be excluded from food preparationand serving areas.

Any food items that have been contaminated should be thrown out.

Any environmental surface suspected of contamination should be promptly cleaned, disinfected with bleach solution, and then rinsed.